Re: [Qemu-devel] [PATCH v7 1/8] virtio: endian-ambivalent targets using legacy virtio

[Greg Kurz]

On Mon, 14 Apr 2014 15:46:34 +0300
"Michael S. Tsirkin" <address@hidden> wrote:

> On Mon, Apr 14, 2014 at 02:40:04PM +0200, Alexander Graf wrote:
> > 
> > On 14.04.14 14:37, Michael S. Tsirkin wrote:
> > >On Mon, Apr 14, 2014 at 02:29:20PM +0200, Alexander Graf wrote:
> > >>On 14.04.14 14:24, Michael S. Tsirkin wrote:
> > >>>On Mon, Apr 14, 2014 at 02:16:03PM +0200, Alexander Graf wrote:
> > >>>>On 14.04.14 13:58, Greg Kurz wrote:
> > >>>>>From: Rusty Russell <address@hidden>
> > >>>>>
> > >>>>>virtio data structures are defined as "target endian", which assumes
> > >>>>>that's a fixed value.  In fact, that actually means it's 
> > >>>>>platform-specific.
> > >>>>>The OASIS virtio 1.0 spec will fix this, by making it all little 
> > >>>>>endian.
> > >>>>>
> > >>>>>We introduce memory accessors to be used accross the virtio code where
> > >>>>>needed. These accessors should support both legacy and 1.0 devices.
> > >>>>>A good way to do it is to introduce a per-device property to store the
> > >>>>>endianness. We choose to set this flag at device reset time because it
> > >>>>>is reasonnable to assume the endianness won't change unless we reboot 
> > >>>>>or
> > >>>>>kexec another kernel. And it is also reasonnable to assume the new 
> > >>>>>kernel
> > >>>>>will reset the devices before using them (otherwise it will break).
> > >>>>>
> > >>>>>We reuse the virtio_is_big_endian() helper since it provides the right
> > >>>>>value for legacy devices with most of the targets, that have fixed
> > >>>>>endianness. It can then be overriden to support endian-ambivalent 
> > >>>>>targets.
> > >>>>>
> > >>>>>To support migration, we need to set the flag in virtio_load() as well.
> > >>>>>
> > >>>>>(a) One solution would be to add it to the stream, but it have some
> > >>>>>     drawbacks:
> > >>>>>- since this only affects a few targets, the field should be put into a
> > >>>>>   subsection
> > >>>>>- virtio migration code should be ported to vmstate to be able to 
> > >>>>>introduce
> > >>>>>   such a subsection
> > >>>>>
> > >>>>>(b) If we assume the following to be true:
> > >>>>>- target endianness falls under some cpu state
> > >>>>>- cpu state is always restored before virtio devices state because they
> > >>>>>   get initialized in this order in main().
> > >>>>>Then an alternative is to rely on virtio_is_big_endian() again at
> > >>>>>load time. No need to mess around with the migration stream in this 
> > >>>>>case.
> > >>>>>
> > >>>>>This patch implements (b).
> > >>>>>
> > >>>>>Note that the tswap helpers are implemented in virtio.c so that
> > >>>>>virtio-access.h stays platform independant. Most of the virtio code
> > >>>>>will be buildable under common-obj instead of obj then, and spare
> > >>>>>some cycles when building for multiple targets.
> > >>>>>
> > >>>>>Signed-off-by: Rusty Russell <address@hidden>
> > >>>>>[ ldq_phys() API change,
> > >>>>>   relicensed virtio-access.h to GPLv2+ on Rusty's request,
> > >>>>>   introduce a per-device is_big_endian flag (supersedes 
> > >>>>> needs_byteswap global)
> > >>>>>   add VirtIODevice * arg to virtio helpers,
> > >>>>>   use the existing virtio_is_big_endian() helper,
> > >>>>>   virtio-pci: use the device is_big_endian flag,
> > >>>>>   introduce virtio tswap16 and tswap64 helpers,
> > >>>>>   move calls to tswap* out of virtio-access.h to make it platform 
> > >>>>> independant,
> > >>>>>   migration support,
> > >>>>>   Greg Kurz <address@hidden> ]
> > >>>>>Cc: Cédric Le Goater <address@hidden>
> > >>>>>Signed-off-by: Greg Kurz <address@hidden>
> > >>>>>---
> > >>>>>
> > >>>>>Changes since v6:
> > >>>>>- merge the virtio_needs_byteswap() helper from v6 and existing
> > >>>>>   virtio_is_big_endian()
> > >>>>>- virtio-pci: now supports endianness changes
> > >>>>>- virtio-access.h fixes (target independant)
> > >>>>>
> > >>>>>  exec.c                            |    2 -
> > >>>>>  hw/virtio/Makefile.objs           |    2 -
> > >>>>>  hw/virtio/virtio-pci.c            |   11 +--
> > >>>>>  hw/virtio/virtio.c                |   35 +++++++++
> > >>>>>  include/hw/virtio/virtio-access.h |  138 
> > >>>>> +++++++++++++++++++++++++++++++++++++
> > >>>>>  include/hw/virtio/virtio.h        |    2 +
> > >>>>>  vl.c                              |    4 +
> > >>>>>  7 files changed, 185 insertions(+), 9 deletions(-)
> > >>>>>  create mode 100644 include/hw/virtio/virtio-access.h
> > >>>>>
> > >>>>>diff --git a/exec.c b/exec.c
> > >>>>>index 91513c6..e6777d0 100644
> > >>>>>--- a/exec.c
> > >>>>>+++ b/exec.c
> > >>>>>@@ -42,6 +42,7 @@
> > >>>>>  #else /* !CONFIG_USER_ONLY */
> > >>>>>  #include "sysemu/xen-mapcache.h"
> > >>>>>  #include "trace.h"
> > >>>>>+#include "hw/virtio/virtio.h"
> > >>>>>  #endif
> > >>>>>  #include "exec/cpu-all.h"
> > >>>>>@@ -2745,7 +2746,6 @@ int cpu_memory_rw_debug(CPUState *cpu, 
> > >>>>>target_ulong addr,
> > >>>>>   * A helper function for the _utterly broken_ virtio device model to 
> > >>>>> find out if
> > >>>>>   * it's running on a big endian machine. Don't do this at home kids!
> > >>>>>   */
> > >>>>>-bool virtio_is_big_endian(void);
> > >>>>>  bool virtio_is_big_endian(void)
> > >>>>>  {
> > >>>>>  #if defined(TARGET_WORDS_BIGENDIAN)
> > >>>>>diff --git a/hw/virtio/Makefile.objs b/hw/virtio/Makefile.objs
> > >>>>>index 1ba53d9..68c3064 100644
> > >>>>>--- a/hw/virtio/Makefile.objs
> > >>>>>+++ b/hw/virtio/Makefile.objs
> > >>>>>@@ -4,5 +4,5 @@ common-obj-y += virtio-bus.o
> > >>>>>  common-obj-y += virtio-mmio.o
> > >>>>>  common-obj-$(CONFIG_VIRTIO_BLK_DATA_PLANE) += dataplane/
> > >>>>>-obj-y += virtio.o virtio-balloon.o
> > >>>>>+obj-y += virtio.o virtio-balloon.o
> > >>>>>  obj-$(CONFIG_LINUX) += vhost.o
> > >>>>>diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > >>>>>index ce97514..82a1689 100644
> > >>>>>--- a/hw/virtio/virtio-pci.c
> > >>>>>+++ b/hw/virtio/virtio-pci.c
> > >>>>>@@ -89,9 +89,6 @@
> > >>>>>  /* Flags track per-device state like workarounds for quirks in older 
> > >>>>> guests. */
> > >>>>>  #define VIRTIO_PCI_FLAG_BUS_MASTER_BUG  (1 << 0)
> > >>>>>-/* HACK for virtio to determine if it's running a big endian guest */
> > >>>>>-bool virtio_is_big_endian(void);
> > >>>>>-
> > >>>>>  static void virtio_pci_bus_new(VirtioBusState *bus, size_t bus_size,
> > >>>>>                                 VirtIOPCIProxy *dev);
> > >>>>>@@ -409,13 +406,13 @@ static uint64_t virtio_pci_config_read(void 
> > >>>>>*opaque, hwaddr addr,
> > >>>>>          break;
> > >>>>>      case 2:
> > >>>>>          val = virtio_config_readw(vdev, addr);
> > >>>>>-        if (virtio_is_big_endian()) {
> > >>>>>+        if (vdev->is_big_endian) {
> > >>>>>              val = bswap16(val);
> > >>>>>          }
> > >>>>>          break;
> > >>>>>      case 4:
> > >>>>>          val = virtio_config_readl(vdev, addr);
> > >>>>>-        if (virtio_is_big_endian()) {
> > >>>>>+        if (vdev->is_big_endian) {
> > >>>>>              val = bswap32(val);
> > >>>>>          }
> > >>>>>          break;
> > >>>>>@@ -443,13 +440,13 @@ static void virtio_pci_config_write(void 
> > >>>>>*opaque, hwaddr addr,
> > >>>>>          virtio_config_writeb(vdev, addr, val);
> > >>>>>          break;
> > >>>>>      case 2:
> > >>>>>-        if (virtio_is_big_endian()) {
> > >>>>>+        if (vdev->is_big_endian) {
> > >>>>>              val = bswap16(val);
> > >>>>>          }
> > >>>>>          virtio_config_writew(vdev, addr, val);
> > >>>>>          break;
> > >>>>>      case 4:
> > >>>>>-        if (virtio_is_big_endian()) {
> > >>>>>+        if (vdev->is_big_endian) {
> > >>>>>              val = bswap32(val);
> > >>>>>          }
> > >>>>>          virtio_config_writel(vdev, addr, val);
> > >>>>>diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > >>>>>index aeabf3a..bb646f0 100644
> > >>>>>--- a/hw/virtio/virtio.c
> > >>>>>+++ b/hw/virtio/virtio.c
> > >>>>>@@ -19,6 +19,7 @@
> > >>>>>  #include "hw/virtio/virtio.h"
> > >>>>>  #include "qemu/atomic.h"
> > >>>>>  #include "hw/virtio/virtio-bus.h"
> > >>>>>+#include "hw/virtio/virtio-access.h"
> > >>>>>  /*
> > >>>>>   * The alignment to use between consumer and producer parts of vring.
> > >>>>>@@ -546,6 +547,8 @@ void virtio_reset(void *opaque)
> > >>>>>      virtio_set_status(vdev, 0);
> > >>>>>+    vdev->is_big_endian = virtio_is_big_endian();
> > >>>>>+
> > >>>>>      if (k->reset) {
> > >>>>>          k->reset(vdev);
> > >>>>>      }
> > >>>>>@@ -897,6 +900,11 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f)
> > >>>>>      BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
> > >>>>>      VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
> > >>>>>+    /* NOTE: we assume that endianness is a cpu state AND
> > >>>>>+     * cpu state is restored before virtio devices.
> > >>>>>+     */
> > >>>>>+    vdev->is_big_endian = virtio_is_big_endian();
> > >>>>>+
> > >>>>>      if (k->load_config) {
> > >>>>>          ret = k->load_config(qbus->parent, f);
> > >>>>>          if (ret)
> > >>>>>@@ -1153,6 +1161,33 @@ void 
> > >>>>>virtio_device_set_child_bus_name(VirtIODevice *vdev, char *bus_name)
> > >>>>>      }
> > >>>>>  }
> > >>>>>+uint16_t virtio_tswap16(VirtIODevice *vdev, uint16_t s)
> > >>>>>+{
> > >>>>>+    if (vdev->is_big_endian) {
> > >>>>>+        return tswap16(s);
> > >>>>>+    } else {
> > >>>>>+        return bswap16(tswap16(s));
> > >>>>>+    }
> > >>>>This looks pretty bogus. When virtio wants to do "tswap" what it
> > >>>>means is "give me a host endianness value in virtio endianness". How
> > >>>>about something like
> > >>>>
> > >>>>#ifdef HOST_WORDS_BIGENDIAN
> > >>>>   return vdev->is_big_endian ? s : bswap16(s);
> > >>>>#else
> > >>>>   return vdev->is_big_endian ? bswap16(s) : s;
> > >>>>#endif
> > >>>>
> > >>>Actually why doesn't this call virtio_is_big_endian?
> > >>>As it is, we get extra branches even if target endian-ness
> > >>>is fixed.
> > >>Because virtio_is_big_endian() returns the default endianness, not
> > >>the runtime endianness of a virtio device.
> > 
> > In fact, we should probably rename it accordingly.
> > 
> > >>
> > >>>>That should work pretty well inline as well, so you don't need to
> > >>>>compile virtio.c as target dependent.
> > >>>>
> > >>>>
> > >>>>Alex
> > >>>Yes but we'll still need to build two variants: fixed endian and
> > >>>dynamic endian platforms.
> > >>>Something along the lines of 32/64 bit split that we have?
> > >>Why bother? Always make it dynamic and don't change the per-device
> > >>variable, no? I'd be surprised if the performance difference is
> > >>measurable. The bulk of the data we transfer gets copied raw anyway.
> > >>
> > >>
> > >>Alex
> > >This will have to be measured and proved by whoever's proposing the
> > >patch, not by reviewers.  Platforms such as AMD which don't do
> > >prediction well would be especially interesting to test on.
> > 
> > Sure, Greg, can you do that? I'm sure Michael has test cases
> > available he can give you to measure performance on this.
> > 
> > Speaking of which, how does all of this work with vhost?
> > 
> > 
> > Alex
> 
> I think that's missing.
> As a first step, we need to disable vhost when
> host/guest endian-ness do not match.
> 
> We could also add cross-endian support to vhost.
> 

I confirm: vhost has to be fixed to use guest endian virtio rings.

> Or just wait a couple more months for virtio 1.0 which is fixed
> endian-ness.
> 

Oohhh no... don't say that ! Legacy virtio is not dead yet.

-- 
Gregory Kurz                                     address@hidden
                                                 address@hidden
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.